1. Field of the Invention
The present invention relates to a new and useful improvement in a process for the isomerization of monocyclic methyl-substituted aromatic hydrocarbon feedstock. The process comprises the use of a specific catalyst in a vapor phase reaction, said catalyst containing a crystalline aluminosilicate zeolite characterized by a constraint index, hereinafter defined, within the approximate range of 1 to 12.
2. Description of the Prior Art
The catalytic rearrangement of alkyl groups present in alkyl aromatic hydrocarbons to provide one or more products suitable for use in the petroleum and chemical industries has heretofore been effected by a wide variety of catalysts. Acidic halides such as aluminum chloride, aluminum bromide, boron trifluoride--hydrogen fluoride mixtures, etc. have been used in the rearrangement of alkyl benzenes to provide valuable intermediates which find utility in the synthesis of rubber, plastic, fibers and dyes. Other catalysts which have been used include solid siliceous cracking-type catalysts such as silica-alumina and clays and platinum deposited on silica-alumina. Although various catalysts possess one or more desired characteristics, a majority of catalysts heretofore employed suffer from several disadvantages. Acidic halides such as aluminum chloride, for example, are partially soluble in the feed material and are easily lost from the catalyst zone. Catalysts of this type are also uneconomical because of their extreme corrosiveness and requirements for recovery from the effluent products. Other catalysts of the heterogeneous type, such as silica-alumina, platinum on alumina, etc., do not possess sufficient acidity to provide effective conversion and necessitate the use of relatively high temperatures above the order of 800.degree. F. to 950.degree. F. High temperatures frequently lead to coke formation which lowers the yield of desired product and necessitates frequent regeneration of the catalyst to remove coke. This results in reducing on-stream time and leads to high catalyst consumption due to loss of catalyst activity. Heterogeneous catalyst such as the crystalline aluminosilicates, both natural and synthetic, possess sufficient acidity but suffer the disadvantage of poor selectivity and aging as evidenced by "coke" make and the excessive amounts of disproportionated product formed in isomerization reactions.
A process in the art for isomerization of xylene is Octafining, extensively discussed in the literature as exemplified by:
1. Pitts, P. M., Connor, J. E., Leun, L. N., Ind. Eng. Chem., 47, 770 (1955). PA0 2. Fowle, M. J., Bent, R. D., Milner, B. E., presented at the Fourth World Petroleum Congress, Rome, Italy, June 1955. PA0 3. Ciapetta, F. G., U.S. Pat. No. 2,550,531. PA0 4. Ciapetta, F. G., and Buck, W. H., U.S. Pat. No. 2,589,189. PA0 5. Octafining Process, Process Issue, Petroleum Refinery, 1st Vol. 38 (1959), No. 11, Nov., p. 278.
The catalyst for use in such process is platinum on silica-alumina.
An improved catalyst for use in Octafining plants is taught by U.s. Pat. No. 3,856,872 to be of the ZSM-5 type of zeolite, whereby the process operates at high space velocities. Further, a process utilizing ZSM-5 type zeolites in acid form for vapor-phase conversion of a feedstock containing mixed C.sub.8 aromatics in the absence of added hydrogen is taught by U.S. Pat. No. 3,856,873. Even in such processes, especially when the catalyst has increased acid activity, there is a loss of xylene, the primary isomerization product, presumably due in part to disproportionation of xylenes and/or transalkylation of xylenes with any ethylbenzene which may be present in the reaction system.
It is hereby proposed and demonstrated that by in situ contacting such a catalyst with a basic nitrogen compound or precursor thereof, loss of primary isomerization product is substantially reduced.
The use of nitrogen compounds for various reasons in hydrocracking and reforming in the presence of catalyst other than that for use herein is known in the art. Examples of such art include U.S. Pat. Nos. 2,849,377 (reforming); 3,657,110 (hydrocracking) and 3,694,345 (hydrocracking). Isomerization of alkyl aromatic compounds in the presence of nitrogen compounds to equilibrium products over catalyst other than that for use herein is taught in U.S. Pat. Nos. 3,293,314 and 3,644,200. The catalyst for use in the process of U.S. Pat. No. 3,293,314 is a solid acidic mixed metal-oxide catalyst, e.g., a silica-alumina mixture, having equilibration (tending to produce equilibrium products) and coke forming activities. Unfortunately, such catalyst materials lose their ability to maintain their equilibration tendency and, therefore, there is a decrease in desirable isomeric products as the catalysts age.